This application is based on Japanese Patent Application No. 2001-27273 filed on Feb. 2, 2001, the content of which is incorporated herein by reference.
1. Field of the Invention
This invention relates to an optical fiber amplifier and an optical communication system using the optical fiber amplifier.
2. Description of the Related Art
It has become increasingly important year after year to increase the capacity of the optical communication system. One of the promising approaches to higher communications capacities is the fiber-optic wavelength division multiplexing system (WDM system). In order to increase the capacity of the WDM system and the number of available channels, an amplifier presenting flat gain characteristics over a wider range of wavelength will be needed; for example, a bandwidth wider than 100 nm is predicted to become necessary in the future.
As conventional amplifiers for the fiber-optic communication system, rare-earth element doped amplifiers such as the Erbium-doped fiber amplifier(EDFA), Thulium-doped fiber amplifier(TDFA) and Praseodymium-doped fiber amplifier(PDFA) have been used. However, the band for signal amplification is dependent on the doped element and this band cannot be changed as desired, in such rare-earth doped fiber amplifiers. In addition, the wavelength range of flat gain is currently limited to 40 nm at the maximum in such rare-earth doped fiber amplifiers. Also, it is not allowed to amplify signals in the 1510-1530 nm range or the range of 1460 nm or less. Three or four rare-earth doped fiber amplifiers for different wavelengths must be jointly used to provide a bandwidth of about 100-200 nm of flat gain characteristics. Then the system will be complex and its manufacturing cost will become high.
Meanwhile, Raman fiber amplifiers have been intensively investigated in recent years, which can amplify light signals in the ranges where the rare-earth doped fiber amplifiers cannot work, and whose amplification range can be set in any desired wavelengths. FIG. 1A illustrates the structure of the prior art Raman amplifier using the silica fiber (hereafter, silica Raman amplifier). The Raman amplifier of this type is described by H. Masuda et al. in Tech. Dig. of ECOC, pp. 139-140, 1998. This amplifier intensifies the input signals that have gone through wavelength division multiplexing. This Raman amplifier has an optical fiber 51 serving as a gain medium, a pump light source 53 for pumping the medium and a coupler 52 for combining the pump light emitted from the pump light source and the signal light. This optical fiber is usually a silica fiber having a large NA(numerical aperture). Note that, for simplicity of description, FIG. 1A does not show common optical parts such as isolators installed before and after the optical fiber.
The amplifier shown in FIG. 1A has the configuration that is most commonly employed where the pump light and the signal light travel in opposite directions, namely, the backward pumping configuration. The following description, however, may apply to the forward pumping configuration as well. The pump light emitted from the pump light source may have a one or more wavelength. FIG. 1B illustrates the gain coefficient spectrum of a silica Raman amplifier using pump light of a single wavelength. The horizontal axis represents the difference in wavelength between the signal light and the pump light. The gain coefficient spectrum of this silica Raman amplifier using pump light of a single wavelength shows a single peak at around 100 nm. The flat gain bandwidth is about 20 nm at most in this silica Raman amplifier using pump light of a single wavelength.
Y. Emori et al. presented a silica Raman amplifier in Proc. of OFC, PD19 in 1999, that was capable for providing a flat gain bandwidth of up to 100 nm by a gain spectrum flattening and bandwidth widening technique using pump light of 10 and some wavelengths. The range of the flat gain bandwidth was determined by the physical properties of the silica fiber. This silica Raman amplifier was very expensive because it needed more than 10 light sources of different wavelengths and an optical circuit for combining the pump light beams emitted from those light sources.
The continuous flat gain bandwidth provided by low-cost amplifiers has been typically limited to about 60 nm in the prior art.
Thus there has been a long-lasting demand for an amplifier capable of providing a wider band (60 nm or more) and flatter gain characteristics than the conventional one, in order to increase the capacity and available channel number of the WDM system.
It is, therefore, the object of the present invention to provide an optical fiber amplifier and an optical communication system using the amplifier for yielding a wideband and flat gain spectrum by combining more than one gain spectrum.
The inventors have found that the gain coefficient spectrum of the Raman amplifier using tellurite-glass as the gain medium (hereafter, tellurite Raman amplifier) lies in longer wavelengths than those for the silica Raman amplifier if the pump wavelength is the same. FIG. 2 shows the gain coefficient spectrum of the tellurite-Raman amplifier using pump light of a single wavelength. The horizontal axis represents the difference in wavelength between the single light and the pump light. As evident from FIG. 2, the tellurite-Raman amplifier has two peaks in its gain coefficient spectrum at around 170 nm and 90 nm in wavelength difference (hereafter, referred to as the first peak P1 and the second peak P2, respectively), while presenting a valley at around 120 nm in wavelength difference (hereafter, the first bottom B1). The gain coefficient falls at wavelengths shorter than the wavelength of the second peak (hereafter, this region is referred to as the second bottom B2).
Since the tellurite Raman amplifier has a Stokes shift larger than that of the silica Raman amplifier and the distance between the first peak P1 and the second peak P2 is long, it has the potential to be a wideband amplifier applicable to wider ranges of wavelength. In order to make the tellurite Raman amplifier available in the WDM system, the gain coefficient spectrum must be flattened by raising the gain coefficient in the first bottom B1 located between the first peak P1 and the second peak P2. Further, if the gain coefficient in the second bottom B2 is also raised, the tellurite Raman amplifier can be used as an amplifier for the WDM system that will utilize a wider bandwidth in the future.
Besides, since the gain coefficient of the tellurite Raman amplifier is higher than that of the silica Raman amplifier, the same level of gain coefficient is provided by a shorter tellurite-glass fiber. For these reasons, the tellurite Raman amplifier is advantageous for use in the WDM system.
The first aspect of the present invention is a Raman amplifier having a tellurite fiber pumped with at least two pump light beams of different wavelengths, wherein the difference in wavelength is predetermined. This Raman amplifier may have two or more tellurite fibers to present a multi-stage structure (the first and second embodiments).
The second aspect of the present invention is a Raman amplifier having a tellurite fiber pumped with pump light of a single wavelength and a silica fiber pumped with another pump light of a single wavelength, wherein the wavelengths of pump light are different from each other (the third to fifth embodiments).
The third aspect of the present invention is a Raman amplifier having a plurality of tellurite and silica fibers alternately located, wherein those fibers are pumped with at least two pump light beams of different wavelengths (the sixth embodiment).
The fourth aspect of the present invention is a Raman amplifier having a tellurite fiber pumped with pump light of a single wavelength and a silica fiber pumped with two or more pump light beams of wavelengths different from each other (the seventh embodiment).
The fifth aspect of the present invention is a Raman amplifier having a tellurite fiber pumped with two or more pump light beams of wavelengths different from each other and a silica fiber pumped with pump light of a single wavelength (the eighth embodiment).
The sixth aspect of the present invention is a Raman amplifier having a tellurite fiber pumped with two or more pump light beams of wavelengths different from each other and a silica fiber pumped with two or more pump light beams of wavelengths different from each other. This Raman amplifier may have an additional tellurite fiber pumped with two or more pump light beams of wavelengths different from each other (the ninth and tenth embodiments).
The seventh aspect of the present invention is a Raman amplifier having a rare-earth doped fiber and a tellurite fiber, wherein those fibers are pumped with pump light beams of wavelengths different from each other (the eleventh to fourteenth embodiments).
The eighth aspect of the present invention is a Raman amplifier having a tellurite fiber to which a low concentration of erbium has been doped, wherein this fiber is pumped with two pump light beams (the fifteenth and sixteenth embodiments).
The ninth aspect of the present invention is an optical communication system having a tellurite fiber and a silica fiber serving as a transmission line, wherein those fibers are pumped with pump light beams of wavelengths different from each other (the seventeenth embodiment).
The tenth aspect of the present invention is an optical communication system having: a tellurite fiber pumped with light of a single wavelength or pumped with two or more pump light beams of wavelengths different from each other; a first silica fiber pumped with light of a single wavelength or pumped with two or more pump light beams of wavelengths different from each other; and a second silica fiber that serves as a transmission line and is pumped with light of a single wavelength or pumped with two or more pump light beams of wavelengths different from each other (the eighteenth embodiment).
The optical fiber amplifier according to the present invention is a Raman fiber amplifier using a tellurite fiber in principle and provides such advantages as the widening of flat gain wavelength region, reduction of noise figure and augmented output of the amplifier, through the combination of techniques: 1) pumping the tellurite fiber with two wavelengths; 2) pumping the tellurite fiber and the silica fiber with different wavelengths; 3) pumping the low-concentration Er-doped tellurite fiber with one or two wavelengths; and 4) pumping the tellurite fiber and the Tm-doped fiber with different wavelengths. The approach of the present invention can reduce the manufacturing cost of the amplifier because the above goals are attained with a minimal number of pumping light sources.
Also the optical communication system according to the present invention is a system that uses a repeater incorporating the tellurite fiber and the silica fiber transmission line for distributive amplification. Through the combination of the above techniques, the range of flat gain bandwidth is expanded, noise figure is reduced, and the output of the amplifier is augmented even with a minimal number of pump light sources.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.